1. Field of the invention
The present invention relates to a system and method for tag estimation and anti-collision of a Radio Frequency Identification (hereinafter, referred to as RFID) system, and more particularly to a system and method for tag estimation and anti-collision in an RFID system, in which an exact number of tags is estimated, information is transmitted based on a Framed-Slotted ALOHA (hereinafter, referred to as FSA) scheme by using an estimated number of tags, and the colliding tags are separated according to the binary tree scheme, thereby achieving rapid identification of the tags.
2. Background Art
As generally known in the art, an RFID system is one of the fields for automatic identification, such as bar codes and magnetic cards, and can be applied to various applied fields, such as personal identification, product identification, location tracking, and banking services. One applied field in which the RFID is most available is the logistics management in which the RFID can replace the existing bar codes.
In a case where a plurality of RFID tags exist in a radio frequency area of a reader in an RFID system, if multiple tags simultaneously transmit their identifier (ID) information to the reader at the time point when the reader wants to identify the tags, collision occurs between the transmitted ID information items, so that the reader cannot identify exact information of the tags. When such collisions increase, the reader requires more time in order to identify all of the tags. Therefore, an effective anti-collision technology is necessary in order to improve the identification speed of the RFID system.
The anti-collision technology is classified in large into an ALOHA-based scheme and a tree-based scheme. The ALOHA-based scheme mainly uses FSA and the tree-based scheme mainly uses the binary tree.
The FSA is a kind of slotted ALOHA scheme, in which time frame is divided into a predetermined number of time slots, and tags access a wireless channel by transmitting ID information based on the time slot unit. Tags generate random numbers within a predetermined frame size and access the time slot according to the order of the random numbers. If multiple tags generate the same random number, the tags access the same time slot and collide with each other. The probability of collision is determined by the number of tags and the size of the frame. Therefore, it is necessary to adjust the size of the frame according to the number of tags to be identified. An FSA having a changing frame size is called a Dynamic framed slotted ALOHA (hereinafter, referred to as DFSA).
The DFSA, an FSA changing the frame size according to situations, may use various methods for determining the frame size, which include a method of increasing the frame size when the collision probability exceeds a predetermined value while decreasing the frame size when the collision probability is below the predetermined value, and a method of estimating the number of tags by using previous frame collision and success probability, etc. and then using an optimum frame size based on the estimation. It is already known in the art that the frame size showing an optimum identification performance is equal to the number of tags.
In the conventional method of estimating the number of tags, the number of tags is calculated and estimated by using the size of a previous frame and the collision probability. The calculation of the number of tags is possible because the collision probability is a function of the frame size and the number of tags. However, when the size of the previous frame is too small in comparison with the actual number of tags, the collision probability of the tags approaches 1. Then, due to the characteristics of the exponential function, the estimation of the number of tags may be either impossible or incorrect. Therefore, the size of the initial frame is important in order to achieve estimation and identification of an exact number of tags. However, there is no reference for determination of the size of the initial frame in the initial stage, and it is thus necessary to optionally determine and use the size of the initial frame.
According to the “EPCTM radio-frequency identification protocols class-1 generation-2 UHF RFID protocol for communications at 860 MHz-960 MHz Version 1.0.9, EPCglobal, January 2005,” which is a standard using the DFSA, tags generate random numbers within a predetermined range and transmit IDs according to the order of the generated random numbers, colliding tags try the transmission at a next frame, and the range of the random number is increased or decreased according to the collision probability, so as to achieve rapid identification.
According to the binary tree scheme, which is another technology for preventing tag collision, all tags transmit their ID information to a reader in the first stage of the identification process. When collision occurs, the colliding tags randomly select 0 or 1 and add the selected value to their own counters, while the tags without relation to the collision unconditionally add 1 to their own counters. Each of the counters has an initial value of 0. The counter decreases each time slot. At the moment when the counter becomes 0, a corresponding tag transmits its ID information to the reader, and the colliding tags are divided into two groups, so as to avoid the collision.
According to the “Information technology automatic identification and data capture techniques—radio frequency identification for item management air interface—part 6: parameters for air interface communications at 860-960 MHz, ISO/IEC FDIS 18000-6, November 2003,” which is an RFID standard using the binary tree scheme, all tags simultaneously transmit data in the initial stage, and the tags are divided into two groups by selecting 0 or 1 when collision occurs. Then, this process is repeated until no collision occurs any more. Then, it is possible to identify all the tags.
One example of such technology for preventing tag collision is disclosed in Korean patent registration No. 0648853 (registered on Nov. 16, 2006 and entitled “Method For Preventing Tag Collision In A Radio Frequency Identification System”).
The method for preventing tag collision technology disclosed in Korean patent registration No. 0648853 includes the steps: (a1) detecting the number of tags causing collision within a Radio Frequency (RF) field; (b1) generating a random number having a short bit when the number of tags is below a predetermined threshold and generating a random number having a long bit when the number of tags exceeds the predetermined threshold; (c1) each tag generating collision preventing ID information having a length of m bits (m≦12) by receiving the random number having a short bit when the random number having a short bit has been generated in step (b1) and generating collision preventing ID information having a length of n bits (12<n≦24) by receiving the random number having a long bit when the random number having a long bit has been generated in step (b1); (d1) accessing each tag and processing data. That is to say, according to the method disclosed in Korean patent registration No. 0648853, the number of tags within the RF area is detected, so that the collision is prevented by using a small random number range when the detected number is below a predetermined value while using a large random number range when the detected number exceeds the predetermined value. Further, ID information items having different lengths are generated according to the number of colliding tags, so as to achieve more rapid data processing. Therefore, the method for preventing tag collision in an RFID system disclosed in Korean patent registration No. 0648853 can sharply increase the number of tags which can be simultaneously processed within a unit time period.
Further, another example of the tag collision preventing technology is disclosed in Korean patent registration No. 0567963 (registered on Mar. 30, 2006 and entitled “Method For Identifying Tags At A High Speed By Using A Division Response Frame ALOHA Scheme In An RFID System”).
The method for identifying tags at a high speed by using a division response frame ALOHA scheme in an RFID system disclosed in Korean patent registration No. 0567963 includes the steps of: (a2) estimating the number of unidentified tags (Tagremain) from responses of RFID tags to a specific number request message of an RFID reader, and then comparing the number of unidentified tags (Tagremain) with a predetermined number of tags (Tagthreshold); (b2) selecting RFID tags capable of responding to the RFID reader by determining the number of division groups and limiting the number of tags responding to the RFID reader when the number of unidentified tags (Tagremain) is larger than the predetermined number of tags (Tagthreshold) (c2) estimating the number of unidentified tags (Tagremain) by receiving responses to the specific number request message of the RFID reader from the RFID tags selected in step (b2); (d2) comparing the number of unidentified tags (Tagremain) with a predetermined number of tags (Tagthreshold) and repeatedly performing steps (b2) and (c2) until the number of unidentified tags (Tagremain) becomes smaller than the predetermined number of tags (Tagthreshold); (e2) when the number of unidentified tags (Tagremain) is smaller than the predetermined number of tags (Tagthreshold), adjusting the size of the frame by determining an optimum frame size, and estimating the number of unidentified tags (Tagremain) by receiving the number of responses of the RFID tags to the specific number request message of the RFID reader; (f2) repeatedly performing step (e2) until the number of unidentified tags (Tagremain) becomes 0. According to the method disclosed in Korean patent registration No. 0567963, tags are divided into a predetermined number of groups and the divided groups are individually identified, when an estimated number of tags is larger than a predetermined value and a maximum frame size is predetermined in the FSA scheme. According to the estimated number of tags, the size of the frame is adjusted or the number of responding tags is limited. As a result, according to the method for identifying tags at high speed by using a division response frame ALOHA scheme in an RFID system disclosed in Korean patent registration No. 0567963, the RFID reader can identify the tags with efficiency over a predetermined value regardless of change in the number of tags.
Another example of tag collision preventing technology is disclosed in a treatise by J. Myung, W. Lee, and J. Srivastava, entitled “Adaptive Binary Splitting for Efficient RFID Tag Anti-Collision” (IEEE Comm. Letter, vol. 10 no. 3 pp. 144-146, March 2006). According to this technology, transmission is based on a binary tree scheme, but tags store the transmission order in a memory without initializing the transmission order, so that the stored transmission order can be used for the next transmission. As a result, this technology can reduce the probability of collision when the same area should be read many times.
Further, another example of tag collision preventing technology is disclosed in a treatise by J. Cha and J. Kim, entitled “Novel Anti-collision Algorithms for Fast Object Identification in RFID System” (in Proc. of Parallel and Distributed System, vol. 2, pp. 63-67, July 2005). This technology proposes a method of estimating the number of tags based on the collision probability according to the FSA scheme and presenting an optimum frame size according to the number of tags.
Moreover, another example of tag collision preventing technology is disclosed in a treatise by S. Lee, S. Joo, and C. Lee, entitled “An Enhanced Dynamic Framed Slotted ALOHA Algorithm for RFID Tag Identification” (in Proc. of MobiQuitous, pp. 166-172, July 2005). According to this technology, tags are divided into a predetermined number of groups and the divided groups are individually identified, when an estimated number of tags is larger than a maximum frame size predetermined based on the FSA scheme.
However, according to the conventional ALOHA scheme, because the initial frame size is optionally fixed, the estimated number of tags is incorrect for the optimum frame size. Even in the case of DFSA using a known optimum frame size, idle time slots are generated within many frames, thereby increasing a waste of channel time during the estimation of the number of tags.
Further, according to the conventional binary tree method, each of the collided groups is always divided into two sub-groups regardless of the number of tags after all tags are transmitted. Therefore, when there are many tags, too many collisions occur in the initial tag identification stage, thereby degrading the identification time performance.
Further, Korean Patent Registration Nos. 0648853 and 0567963 fail to disclose a detailed method for detecting the number of tags.
Moreover, the method disclosed in the treatise by J. Myung, W. Lee, and J. Srivastava cannot show a good performance because the initial tag identification is achieved in completely same way as that of the binary tree method. Moreover, this method can have a good performance in the reading after the initial stage only when there are a similar number of tags within the area.
In addition, the method disclosed in the treatise by J. Cha and J. Kim is relatively exact when the collision probability is low. However, as the collision probability approaches 1, it is impossible to achieve exact estimation. That is to say, for the first frame, it is impossible to determine the frame size through estimation, and it is thus inevitable to use a fixed initial frame size. When the fixed initial frame size is much smaller than the actual number of tags, the collision probability approaches 1 and it is thus impossible to estimate the exact number of tags.
Also, according to the method disclosed in the treatise by S. Lee, S. Joo, and C. Lee, the estimation of the number of tags becomes incorrect when the collision probability is high.